Update sk-config example applications to show serial-to-serial communication

This change adds a third cell and refactors the system so that user
input originating from UART2 on the Sabre Lite is read by cell 1,
processed (and potentially modified) by cell 2, and emitted by cell
3 on UART1.
loss detection?

Did you play with scheduleRate and runtime to see if you could get a schedule generated that ran the receiver more often than the sender so that loss was (statistically speaking) avoided?